DESCRIPTION: Cell-based transmission of HIV is far more efficient than the transfer of cell-free virus, and is likely to play a prominent role in HIV transmission when infected cells enter the bloodstream, as can occur during the intravenous injection of drugs by substance abusers. This may complicate vaccination efforts, because cell-based transmission allows HIV to evade neutralizing antibodies. An understanding of the molecular mechanisms underlying cell-to-cell transmission thus appears particularly relevant for the prevention of HIV transmission among drug abusers. We have identified the host proteins kindlin-3 and pacsin2 as novel binding partners HIV-1 Gag. Both proteins are dispensable for the completion of a full replication cycle after infection with cell-free virus. Nevertheless, both are essential for the spreading of HIV-1 among T cells, indicating that kindlin-3 and pacsin2 are critical for the cell-to-cell transmissionof HIV-1. Kindlin-3 is required for the function of integrins such as LFA-1, which has been implicated in HIV transmission. However, our data demonstrate a striking requirement for kindlin-3 for HIV-1 replication even in T lymphoid cells lacking LFA-1. Our working model is that HIV-1 Gag mimics the kindlin-recruiting activity of integrins to induce donor cell polarization towards the virological synapse and thereby promote virus transfer. Pacsin2 can generate membrane curvature and nucleate filopodia formation. HIV-1 frequently buds from filopodia, and viral filopodia capped by Gag have been implicated in HIV-1 cell-cell transmission. Thus, pacsin2 may be important for HIV-1 transmission because it is involved in viral filopodia formation. The goal of the project is to understand the roles of kindlin-3 and pacsin2, and of the cellular pathways in which these proteins are known to function, in the spreading of HIV-1. We also propose to investigate whether these pathways are altered among a cohort of elite controllers that includes drug abusers. The planned research may yield important translational insights into how to reduce HIV transmission among drug abusers.